500g/1lb 2oz Maris piper potatoes, peeled and roughly chopped
75ml/2½fl oz full-fat milk
75ml/2½fl oz full-fat double cream
75g/2¾ oz butter
1 bay leaf
2 garlic cloves, crushed
½ small bunch thyme
salt and freshly ground black pepper
For the sautéed leeks
1 tbsp olive oil
1 leek, trimmed and finely chopped
1–2 thyme sprigs
To make the cod, preheat the oven to 220C/200C Fan/Gas 7.
Roll the slices of bread thinly using a rolling pin and trim to fit the size of the cod fillets. Mix the prawns and lemon juice together in a bowl. Spread half the prawns on each slice of bread and lay the bread on the cod, prawn-side down. Transfer to the fridge to chill for 30 minutes.
Heat the oil in a frying pan and add the cod, bread-side down to the pan. Cook until the bread is lightly golden, then flip over and transfer to a baking tray, cod-side down. Roast for 6–8 minutes, or until the cod is cooked through. Leave to rest.
Meanwhile, to make the buttery mash, place the potatoes in a large saucepan of water over a high heat and bring to the boil. Reduce the heat and simmer for 15 minutes, or until the potatoes are tender. Drain in a colander and leave until all the steam has evaporated. Mash lightly and then push through a fine sieve or potato ricer.
Place the milk, cream, butter, bay leaf, garlic and thyme in a large saucepan over a low heat and simmer gently for 20 minutes. Strain with a sieve and beat the milk into the mashed potatoes. Season with salt and pepper.
To make the sautéed leeks, heat the oil and butter in a small frying pan and add the leeks and thyme. Fry for 4–5 minutes until golden-brown.
Spoon the mash onto warmed plates, top with the leeks and finish with the cod. Serve with the lemon wedges.